A ceramic honeycomb structural body has been widely used as a filter that traps a particulate matter included in exhaust gas from, for example, an internal combustion or a boiler, or a catalyst carrier of an exhaust gas purification catalyst. The honeycomb structural body is a tubular (for example, cylindrical) structural body whose both ends are open and which includes inside what is called a honeycomb structure (honeycomb construction). In other words, the honeycomb structural body includes a plurality of cells in the inside thereof surrounded by a tubular outer surface (outer wall), the plurality of cells being partitioned by a partition and each being aligned with an axial direction of the structural body. The ceramic honeycomb structural body, which is excellent in thermal resistance, thermal shock resistance, and oxidation resistance, provides wide usage in addition to the above-described usages.
Typically, the ceramic honeycomb structural body is manufactured by shaping, by an extrusion molding method, a clay body obtained through mixing of ceramic (for example, alumina) powder as a component material thereof together with organic binder, water and the like to obtain a honeycomb compact, and then by firing the honeycomb compact thus obtained.
However, when the honeycomb structural body is manufactured by this method, some defects such as occurrence of a crack on the outer wall of the honeycomb structural body and adhesion of a foreign substance to the outer wall may occur. The occurrence of a crack and the adhesion of a foreign substance potentially cause, for example, reduction in the strength of the honeycomb structural body, reduction in filtering performance when the honeycomb structural body is used as a filter, reduction in exhaust-gas purification performance when the honeycomb structural body is used as a catalyst carrier. Thus, inspection of the presence or absence of these defects needs to be performed before using the honeycomb structural body. Technologies for such defect inspection have been publicly known (for example, refer to Patent Documents 1 to 3).
Among these technologies, Patent Documents 1 and 2 each disclose a method of performing image capturing of the surface of an outer wall, as a side surface, of a cylinder honeycomb structural body being rotated about a central axis thereof, and inspecting any defect on the outer wall based on a result of the image capturing
The existence of a defect as described above affects a characteristic of the honeycomb structural body, and thus the defect inspection is important to maintain the quality of the honeycomb structural body. In particular, when the cylindrical honeycomb structural body is manufactured as a ceramic fired body by firing a honeycomb compact obtained through extrusion shaping, a crack is likely to be formed along the axial direction of the honeycomb structural body, and it is required to excellently detect the crack of that kind.
On the other hand, in the cylindrical honeycomb structural body as a ceramic fired body, an undulation (surface relief) exists along a circumferential direction on the outer wall thereof in some cases. This undulation does not affect the quality and characteristic of the honeycomb structural body, and thus the existence thereof is allowed.
However, there is such a problem that, when the defect inspection is performed by, for example, the methods disclosed in Patent Documents 1 and 2, what is called over-detection (excessive detection) occurs that an undulation, which does not need to be determined as a defect, is detected as a defect.
In particular, a shadow formed at a crack extending in the axial direction of the honeycomb structural body and a shadow formed at a recess formed in the axial direction of the honeycomb structural body due to a generated undulation are similar, and the over-detection of the latter as the former is likely to occur.
Visual inspection has a high crack detection reliability but takes a long inspection time, and thus is disadvantageous in production efficiency and cost.